In graphic arts applications three developing systems are commonly used. Classical "lith" developers, conventional "rapid-access" developers and "hard-dot" rapid access systems which are also referred to as "hybrid" systems.
The mechanism for development of the three systems are distinct and account for their unique characteristics. The chart below illustrates the benefits and weaknesses of each development system.
__________________________________________________________________________ CHARACTERISTIC LITH RAP HYBRID __________________________________________________________________________ RATE OF DEVELOPMENT SLOW FAST FAST STABILITY OF CHEMISTRY PODR GOOD MEDIOCRE* DOT QUALITY EXCELLENT POOR EXCELLENT DEVELOPMENT LATITUDE POOR GOOD GOOD COMPATIBLE EMULSIONS HIGH CL- ONLY ALL ALL SCREEN RANGE POOR WIDE MEDIUM AUXILIARY DEVELOPER NO YES YES __________________________________________________________________________ *Systems at lower pH are more stable
"Lith" developers are characterized by a low free sulfite content and the application of hydroquinone as the sole developing agent. This leads to development systems which produce sharp edged pictures on halftone and line copies.
The lith chemistry is referred to as an infectious developer (i.e. a cyclic mechanism that increases efficiency) but in reality it is not, and has the following mechanism:
(1) HQ+AgX(EXPOSED).fwdarw.Q+Ag.sup.o +HX PA1 (2) Q+HQ.fwdarw.2SQ.sup.o PA1 (3) SQ.sup.o +AgX(EXPOSED).fwdarw.Ag.sup.o +Q +X.sup.- PA1 (1) HQ+AgX(EXPOSED).fwdarw.Q+Ag.sup.o +HX PA1 (2) Q+Phen.sup.- .fwdarw.HQ+Phen.sup.o PA1 (1) HQ+AgX(EXPOSED).fwdarw.Q+Ag.sup.o +HX PA1 (2) Q+Nucleator.fwdarw.Oxidized Nucleator PA1 (3) Oxidized Nucleator+OH.sup.- .fwdarw.Hydrolyzed Nucleator PA1 (4) Hydrolyzed Nucleator+AgX(EXPOSED.fwdarw.Ag.sup.o +Byproducts or UNEXPOSED)
The quinone (Q) from equation 3 recycles into equation 2. Inclusion of low amounts of free sulfite into the compositions as a preservative is optional. Typically lith films contain high chloride emulsions (at least about 60% by weight silver chloride based on total silver halide). The chemical stability of the lith chemistry is poor due to the low concentrations of sulfite and needs constant monitoring.
"Rapid access" developers are formulated by a combination of two superadditive developing agents and high sulfite content. This leads to their practical advantage of wide processing latitude, (i.e. development time and/or temperature) and excellent chemical stability of the developer solutions. These advantages however are at the expense of superior dot and line quality.
The Rapid Access Process (RAP) developer chemistry has the following mechanism:
(3) Q+SO.sub.3.sup.= .fwdarw.HQSO.sub.3.sup.-
The hydroquinone (HQ) reacts with the silver-halide when exposed to light to form elemental silver resulting in the formation of the latent image on the photographic element. The compositions include a relatively high sulfite content which reacts with the free quinone (Q) to form a sulfonated hydroquinone. The free quinone also reacts in the presence of the "auxiliary developer" i.e. phenidone (Phen.sup.-) to regenerate hydroquinone. Other hydrazine compounds, including pyrazolidone compounds, and amino phenols can be used as "auxiliary developers" in RAP development systems.
The "hard-dot" rapid access or "hybrid" systems combine the good features of both rapid-access and lith developers into one formula. On the one hand lith-like halftone dot acutance has been obtained by means of an infectious developer. On the other hand rapid access is achieved by a system that maintains its stability and consistency by means of a single replenisher and can be run in simple, compact and inexpensive machines. This is achieved by combining a high sulfite content and an auxiliary developing agent. Possible mechanisms for this hybrid system can be based on, but are not restricted to hydrazine or tetrazolium salt chemistries which are also referred to as "nucleators"These systems have the superior quality of classical "lith" systems in addition to the good chemical stability of the conventional rapid-access systems.
Initially, the hybrid system required extremely high chemistry pH, an amine booster and a nucleator and therefore films that were designed for this chemistry could not be used in any other system. These drawbacks are being eliminated as more active nucleators are designed that allow processing at lower pH. Such nucleators are described in U.S. Pat. No. 4,269,929 (to Nothnagle) and U.S. Pat. Nos. 4,997,980, 4,686,167 and 5,220,022 (to Resnick et al.). The current family of nucleators, as described in British application 94104254 (Fryberg) filed May 24, 1994 and British application 94104007 (Fryberg) filed May 24, 1994, has sufficient activity to allow processing not only in hydroquinone but also the ascorbic acid developing systems as used in the invention.
The hybrid chemistry is truly an infectious developer in that it can develop unexposed silver halide, unlike the lith system and this accounts for the concern for peppering. The hybrid chemistry has the following mechanism:
The hydrolyzed nucleators are so active that they can be non-discriminating between the exposed and unexposed silver halide. Thus without the incorporation of restrainers or like materials and controlled process parameters, using these developing compositions can result in overdevelopment and "high fog".
In summary, the disadvantages of the lith chemistry--poor stability, high replenishment rate, poor development rate and latitude are the overriding factors in looking for alternative systems. The Rapid Access Process (RAP) chemistry, essentially eliminated these defects, but significantly deteriorated dot image quality making it unusable for generating screened halftone negatives. The hybrid system, which combines the best features of the lith and RAP technologies is the preferred system over the other systems. The ascorbic acid developing compositions provided by the present invention are effectively and preferably used in both RAP and hybrid systems.
In all three development systems, generally hydroquinones are used as the developing agent. Photographic developer compositions containing hydroquinones as developing agents are well known in the art. However due to the inherent toxicity and environmental hazards posed by hydroquinone or its derivatives, alternate developer compositions having less toxic substitutes are desirable. In replacement of the hydroquinone developing agents the art has shown use of ascorbic acid and related variations of ascorbic acid.
In particular, U.S. Pat. No. 2,688,549 to James et al. discloses a photographic developer composition and methods to accelerate the rate of developing agents. The developing agents include non-aromatic ascorbic acid and sugar analogs which contain mono-nuclear heterocyclic ene-diol groups. The developing process is accelerated by the presence of auxiliary developing agents, i.e. 1-phenyl 3-pyrazolidone.
U.S. Pat. No. 3,826,654 to Weiss et al. discloses a composition and process for developing surface latent and internal latent images. The developer composition has a pH of at least 12 and contains a 3-pyrazolidone derivative, ascorbic acid, a thione/thiol substituted nitrogen containing heterocyclic compound and alkali metal iodide.
U.S. Pat. No. 5,196,298 to Meeus et al. discloses a photographic developing composition for immersion development having a pH of at least 12 containing more than 0.45 mole/liter, preferably 0.4 to 0.9 mole/liter, of an ascorbic acid developing agent.
James, Weiss and Meeus all disclose photographic developer compositions free of hydroquinones and include ascorbic acid as a developing agent. However, Weiss and Meeus do not contain a sulfite component and the presence of sulfite is optional in James. It is advantageous to provide sulfite in the developer compositions to maintain developing activity of the compositions upon storage and reportedly acts as a silver halide sludge inhibitor during the processing of silver halide film.
U.S. Pat. Nos. 5,098,819 and 5,278,035 to Knapp disclose developing compositions including ascorbic acid as a developing agent and a sulfite component. In particular, Knapp U.S. Pat. No. 5,098,819 discloses a developer composition having a pH from 9.75 to 10.6 consisting of ascorbic acid and sugar-type derivatives thereof, their salts and mixtures thereof; a sulfite in an amount of from 2 to 20 g; an alkali metal carbonate in an amount of from 15 to 30 g; and a 3-pyrazolidone compound and water to 1.0 liter. For rapid access and hybrid systems these compositions provide poor oxidation and rates of development and in hybrid systems also provide poor dot quality.
Knapp U.S. Pat. No. 5,278,035 is limited to a non-toxic photographic developer composition for processing x-ray films in automatic film processors. Knapp '035 discloses a composition comprising a non-hydroquinone and non-alkali metal hydroxide developer for radiographic materials and a method of developing. The radiographic developer composition has a pH from 9.5 to 10.6 and comprises at least 15 grams of ascorbic acid, 2-50 grams of sulfite, a 3-pyrazolidone compound, and 35-90 grams of an "anti-swelling" agent such as sodium sulfate, glutaraldehyde or mixtures thereof and water to 1.0 liter.
U.S. Pat. Nos. 5,236,816 and 5,264,323 to Purol et al. disclose photographic developing compositions including ascorbic acid as a developing agent. The Purol U.S. Pat. No. 5,236,816 discloses a solution having a pH from 9.5 to 11.5, comprising an ascorbic acid developing agent, an auxiliary super-additive developing agent, (i.e. 3-pyrazolidone) and a carbonate buffering agent in a concentration of at least 0.5 molar. Purol U.S. Pat. No. 5,264,323 is directed to a process for forming high contrast photographic images. The process includes exposing a silver halide photographic element containing a hydrazine and amino compound component and developing in the solution described in patent '816.
The compositions in Purol are not directly dependent on the use of sulfites, which are only optionally included in the compositions at a moderate level as a preservative. In rapid access and hybrid development systems the absence of a sulfite component provides good dot quality, however stability of these compositions are limited and tend to discolor upon exposure to air.
The photographic developing composition of the present invention has a pH between 10.1 and 10.9 and contains at least 0.17 moles per liter of an ascorbic acid developing agent, 0.3 to 0.5 moles per liter of a sulfite and 0.2 to 0.4 moles per liter of a carbonate buffer. Advantage over prior art compositions are provided by the particular combination and concentrations of these constituents. The present invention is directed to the provision of such a non-toxic developing composition which may be used to effectively process both hybrid and rapid access film/paper products and provide good dot quality, greater stability and greater capacity than known developers.
Accordingly, it is a broad object of the invention to provide a photographic developing composition that is free of dihydroxybenzene developing agents and their use in the processing of photographic elements.
A more specific object of the invention is to provide a composition that includes a non-toxic developing agent such as ascorbic acid which provides environmental advantages over hydroquinone developing agents.
Another object of the invention is to provide a method that utilizes the developer compositions in processing hybrid and rapid access film/paper products.
Another more specific object of the invention is to provide a photographic development method which is an improvement over known methods, in particular the developing compositions used have a greater aerial oxidation stability, require low replenishment and provide greater capacity for developing film before exhausting.
A further specific object of the invention is to provide an ecologically safe method for processing photographic elements using the developer compositions of the invention and fixer solutions that are either ammonia-free or of a low-ammonia content.